Digital to analog converter with passive reconstruction filter

ABSTRACT

A DAC design uses a passive reconstruction filter. The reconstruction filter includes a notch filter and series peaking filter (low pass filter with peaking in the signal passband). The notch filter provides notch filtering at the DAC clock frequency. The peaking filter increases signal bandwidth while attenuating frequency contents at harmonics of the DAC clock frequency. The notch filter can be an LC notch filter with at least one notch inductor Ln and at least one notch capacitor Cn. The peaking filter can be a series peaking inductor Ls (shunted with a filter capacitor Cp). In a differential configuration, the passive reconstruction filter can be configured with ±LC notch filters (with ±Ln notch inductors), and the peaking filter can be ±Ls peaking inductors coupled in series to the ±LC notch filters. The ±Ln notch inductors, ±Ls peaking inductors can be mutually wound as single inductors. For an example direct conversion RF transmit chain, IQ± signal paths are implemented with differential DAC designs including passive reconstruction filters.

CROSS-REFERENCE TO RELATED APPLICATIONS

Priority is claimed under 37 CFR 1.78 and 35 USC 119(e) to U.S. Provisional Application 62/279183 (Docket TI-76393), filed 15 Jan. 2016), which is incorporated by reference.

BACKGROUND

Technical Field

This Patent Disclosure relates generally to digital to analog converters.

Related Art

Digital to Analog Converters (DACs) convert digital data into a sequence of impulses that are then processed by a reconstruction filter to band limit the DAC output, removing spurious high-frequency content. Reconstruction filters are also referred to as anti-imaging filters in that they prevent imaging by smoothing the DAC output waveform to remove image frequencies above the Nyquist limit. For example, in the case of RF transmitter applications, reconstruction filters remove DAC images prior to upconversion.

Reconstruction filters can be implemented with active RC filter circuits, avoiding the use of external inductors. Disadvantages of active reconstruction filters include power consumption, and sub-optimal noise and linearity.

BRIEF SUMMARY

This Brief Summary is provided as a general introduction to the Disclosure provided by the Detailed Description and Drawings, summarizing aspects and features of the Disclosure. It is not a complete overview of the Disclosure, and should not be interpreted as identifying key elements or features of, or otherwise characterizing or delimiting the scope of, the disclosed invention.

The Disclosure describes apparatus and methods for a digital to analog converter design with passive reconstruction filter, such as for use in a direct conversion RF transmitter with IQ± signal paths.

According to aspects of the Disclosure, a DAC design uses a passive reconstruction filter. The reconstruction filter includes a notch filter and series peaking filter (low pass filter with peaking in the signal passband). The notch filter provides notch filtering at the DAC clock frequency. The peaking filter increases signal bandwidth while attenuating frequency contents at harmonics of the DAC clock frequency. The notch filter can be an LC notch filter with at least one notch inductor Ln and at least one notch capacitor Cn. The peaking filter can be a series peaking inductor Ls (shunted with a filter capacitor Cp). In a differential configuration, the passive reconstruction filter can be configured with ±LC notch filters (with ±Ln notch inductors), and the peaking filter can be ±Ls peaking inductors coupled in series to the ±LC notch filters. The ±Ln notch inductors, ±Ls peaking inductors can be mutually wound as single inductors. For an example direct conversion RF transmit chain, IQ± signal paths are implemented with differential DAC designs including passive reconstruction filters.

Other aspects and features of the invention claimed in this Patent Document will be apparent to those skilled in the art from the following Disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example embodiment of a digital to analog converter (DAC) design [10], including a DAC [12] and a passive reconstruction filter [14/16] according to this Disclosure.

FIG. 2 illustrates an example embodiment of a differential DAC [210] with a passive reconstruction filter [214/216], implemented in the differential IQ± signal paths of a direct conversion RF transmitter design [200].

FIG. 3 illustrates an example direct conversion TX/RX transceiver design [300], including dual TX signal chains [301/302], each including differential IQ± signal paths implemented with a DAC [312] and passive reconstruction filter [314/316] according to this Disclosure.

DETAILED DESCRIPTION

This Description and the Drawings constitute a Disclosure for [XXXX], including describing example embodiments, and illustrating various technical features and advantages.

In brief overview a DAC design uses a passive reconstruction filter. The reconstruction filter includes a notch filter and series peaking filter (low pass filter with peaking in the signal passband). The notch filter provides notch filtering at the DAC clock frequency. The peaking filter increases signal bandwidth while attenuating frequency contents at harmonics of the DAC clock frequency. The notch filter can be an LC notch filter with at least one notch inductor Ln and at least one notch capacitor Cn. The peaking filter can be a series peaking inductor Ls (shunted with a filter capacitor Cp). In a differential configuration, the passive reconstruction filter can be configured with ±LC notch filters (with ±Ln notch inductors), and the peaking filter can be ±Ls peaking inductors coupled in series to the ±LC notch filters. The ±Ln notch inductors, ±Ls peaking inductors can be mutually wound as single inductors. For an example direct conversion RF transmit chain, IQ± signal paths are implemented with differential DAC designs including passive reconstruction filters.

FIG. 1 illustrates an example embodiment of a DAC (digital to analog converter) design 10, including a DAC 11 and a passive reconstruction filter 12 according to this Disclosure.

The passive reconstruction filter is configured with a notch filter 14 and series peaking filter 16 (low pass filter with peaking in the signal passband). The order of the notch filter and the peaking filter is a design choice. The use of multiple successive notch filters is a design choice. For the example differential DAC design, the reconstruction filter includes ±LnCn notch filters, and series ±Ls peaking inductors, with a shunt filter capacitor Cp referred to a circuit common.

The notch filter suppresses DAC output images. The peaking filter provides bandwidth enhancement using inductive peaking while attenuating frequency contents at harmonics of the DAC clock frequency. That is, the passive reconstruction filter (notch filter with peaking circuit) is able to suppress DAC images while maximizing signal bandwidth.

FIG. 2 illustrates an example embodiment of a DAC design 210 implemented in the differential IQ± signal paths of a direct conversion RF transmitter design 200. The differential DAC design 210 includes a DAC 211 and a passive reconstruction filter 212, with ±LnCn notch filters, and series ±Ls peaking inductors. For the example implementation, the notch filters 214 are connected to DAC outputs, followed in the signal path by the series peaking inductors 216.

The RF transmitter design includes upconversion mixers 220 driven by LO (local oscillator) signals LO IQ±, and TX± PPAs (pre-power amplifiers) 231/232, driving out IQ TX RF signals through an external match circuit 240.

For the example RF application, the passive reconstruction filter design takes advantage of the use of oversampling to bring down quantization noise, so that the DAC images are at a frequency significantly higher than baseband, facilitating the use of integrated inductors. The LC notch filter is used to suppress these images, but they impact band droop. The peaking inductor is used to counter the band-droop effects of the LC notch filter, enhancing bandwidth.

For the example differential DAC design, in which the passive reconstruction filter is constructed with ±LnCn notch filters and ±Ls peaking inductors), takes advantage of mutual coupling for the configuration of the inductors. The four inductors for the differential implementation (two for notching and two for peaking) are implemented using two mutually wound inductors. This configuration reduces filter size, and can enhance filter performance.

That is, the example embodiment of a DAC design with passive reconstruction filter is able to take advantage of mutual inductance so that the four inductors (±Ln and ±Ls) become two. The notch inductors ±Ln are inter-wound to have positive mutual coupling, providing signal cancellation, enhancing the notch filtering and providing more rejection at the notch frequency. The peeking inductors and ±Ls are inter-wound to have negative mutual coupling, enhancing the signal (V positive−V negative) by enhancing peeking filtering. So the net transfer function is such that the droop of the in-band signal is reduced, and rejection of the unwanted signal (in this case DAC image that is at the DAC clock frequency) is increased.

The use of a passive reconstruction filter according to this Disclosure: (a) allows for a narrower transition band (signal bandwidth and DAC images can be much closer) compared to traditional active filter implementations; (b) since only passive elements are used, DC power consumption is eliminated; and (c) passive elements exhibit high linearity while contributing minimum noise and minimum in band insertion loss.

The Disclosure provided by this Description and the Figures sets forth example embodiments and applications illustrating aspects and features of the invention, and does not limit the scope of the invention, which is defined by the claims. Known circuits, connections, functions and operations are not described in detail to avoid obscuring the principles and features of the invention. These example embodiments and applications, including design examples, can be used by ordinarily skilled artisans as a basis for modifications, substitutions and alternatives to construct other embodiments, including adaptations for other applications. 

1. A circuit suitable for analog to digital signal conversion, comprising a digital-to-analog converter (DAC) coupled to receive input digital data, and configured to convert the digital data to a DAC output; a reconstruction filter constructed with passive components to provide a converted signal at an output, including at least one notch filter to provide notch filtering, and a series peaking filter coupled to the notch filter to increase signal bandwidth for the converted signal.
 2. The circuit of claim 1, wherein the notch filter comprises an LC notch filter with at least one notch inductor Ln and at least one notch capacitor Cn components.
 3. The circuit of claim 2, wherein the converted signal is a differential converted signal, and the DAC comprises a differential DAC configured to convert the input digital data into differential DAC outputs +DAC and −DAC; the notch filter comprises a +LC notch filter coupled between the +DAC output and a circuit common, and a −LC notch filter coupled between the −DAC output and the circuit common; and the peaking circuit comprises a peaking inductor +Ls coupled to the +LC notch filter, and a peaking inductor −Ls coupled to the −LC notch filter.
 4. The circuit of claim 3, wherein the ±LC notch filters respectively include a ±Ln inductors, and the peaking inductors ±Ls; and wherein the ±Ln notch inductors and the ±Ls peaking inductors are respectively mutually wound as single inductors.
 5. The circuit of claim 3, adapted for use in a direct conversion RF transmit chain including an IQ± signal paths: wherein the I-Path includes an I-Path DAC and I-Path reconstruction filter providing a converted I-Path signal with I± signal outputs; and wherein the Q-Path includes a Q-Path DAC and Q-Path reconstruction filter providing a converted Q-Path signal with Q± signal outputs.
 6. The circuit of claim 1, wherein the DAC and the reconstruction filter are integrated into a single integrated circuit. 